1. Field of the Invention
This invention relates to a wireless communication system and a terminal device suitable for use in wireless LAN (local area network) connecting a plurality of terminal devices without lines.
2. Description of Related Art
Along with movements toward higher performances of computers, it is actively practiced to establish LAN by connecting a plurality of computers to share files and data or to transfer e-mails and data. In conventional LAN, computers are wire-connected by using optical fibers, coaxial cables, or twisted pair cables. However, wire LAN needs a wire connection work. Therefore, it is difficult to easily establish LAN, and cables extend complexly.
Thus, wireless LAN is being remarked as a system releasing users from wiring of conventional wire LAN. Recently, in particular, the use of information as multi-media has been progressed, and data having a large quantity such as image data and audio data and requiring high-speed transmission have come to be treated frequently. Taking it into consideration, the interface of IEEE1394 is being remarked, and there is a demand for wireless LAN available for wireless transmission of IEEE1394 data.
In case of establishing wireless LAN, wireless communication units are connected to computers and peripheral devices. The wireless communication unit transmits and receives data modulated by a predetermined modulation system to enable wireless transmission of data among wireless communication terminal devices, and controls the transmission procedure and the transmission timing of data among wireless communication terminal devices. CSMA/CA (carrier sense multiple access with collision detection) system is known as a system for controlling wireless LAN.
Conventionally, in such wireless LAN (local area network), in the case where a plurality of wireless communication terminal devices exist, since received signals from wireless communication terminal devices vary in signal intensity, it is necessary to re-adjust AGC (automatic gain control) for every wireless communication terminal device of a partner for exchanging data with. Conventionally, therefore, a symbol for AGC control is provided at a predetermined position in a frame to perform AGC control by detecting the symbol.
That is, FIG. 14 shows the structure of a single frame in a conventional wireless LAN. As shown in FIG. 14, a symbol for AGC is provided at the forefront of a frame. It is followed by start symbols and a unique word, and data is transmitted next to them.
In a wireless communication unit forming each wireless communication terminal device, when a signal is received from a wireless communication terminal device, the signal level of the received signal of the symbol for AGC at the forefront of a frame is detected. Then, the gain for the received signal is controlled so that the signal level of the received signal is a predetermined level. Controlled by such AGC control to equalize the signal level of the received signal to the predetermined level, data reception is executed.
In this manner, in conventional wireless LAN, the wireless communication unit forming each wireless communication terminal device needs the time for pulling in the AGC loop every time when a signal is received. This time not only results in wasting wireless resources but also invites an increase of the transmission delay time.
Especially, wireless LAN designed for wireless transmission of IEEE1394 data, a high speed is required. Therefore, it is desirable to reduce the time for AGC control and to increase the response speed. Additionally, the existence of the special symbol for ACE inevitably increases the overhead, and it is therefore desirable to omit the such special symbols.
It is therefore an object of the invention to provide a wireless communication system and a terminal device capable of increasing the response speed of an AGC circuit thereof to signals from a plurality of wireless communication terminal devices.
According to the invention, there is provided a wireless communication system having a plurality of terminal devices to make communication among these o terminal devices in a time sharing fashion, in which each terminal device comprising: AGC control means for controlling so that the level of a received signal be a predetermined level; control value storage means for maintaining control values of the AGC control means for individual terminal devices under transmission; and transmitting terminal device identifying means for identifying a terminal device under transmission, the transmitting terminal device identifying means identifying a terminal under transmission, a control value corresponding to the terminal device under transmission being read out from the control value storage means and used as the control value of the AGC control means.
According to the invention, there is further provided a terminal device comprising: AGC control means for controlling so that the level of a received signal be a predetermined level; control value storage means for storing a control value of the AGC control means for each terminal device under transmission; and transmitting terminal deice identifying means for identifying a terminal under transmission, the transmitting terminal device identifying means identifying the terminal under transmission, the control value corresponding to the terminal device under transmission being read out from the control value storage means and used as the control value of the AGC control means.
According to the invention, there is further provided a terminal device comprising: a plurality of antenna means;
antenna switching means for switching received outputs of the plurality of antenna means; AGC control means for controlling so that the level of a received signal be a predetermined level; and control value storage means for storing a control value of the AGC control means for each of the plurality of antennas, an antenna currently receiving a signal being identified from the plurality of antennas, one of control values corresponding to the antenna currently receiving the signal being read out from the control value storage means and used as the control value of the AGC control means.
For each terminal device currently under transmission, the AGC control value is stored in RAM. Thus, the terminal device currently engaged in transmission is judged, and a control value for the terminal device under transmission is read out from RAM to effect AGC control. As a result, even when the terminal device under transmission changes from one to another, the AGC loop can be followed at a high speed. Additionally, according to the invention, no special area for AGC is required in each frame, and the overhead is improved. Furthermore, when a diversity antenna is used, the AGC loop can be followed by storing AGC control values of individual antennas in RAM.